1. Field of the Invention
The invention relates to a signal transmission system comprising processing means for isolating an estimate for at least one wanted signal contained in at least one mixed signal, at least one sensor for detecting the mixed signal, the mixed signal comprising at least the wanted signal and at least two correlated interference signals which are produced by two sources of the system in response respectively to two correlated electric signals.
This signal transmission system may in turn relate to an audio signal broadcasting system present, for example, in a motor car or in a room. The system comprises a sound source formed, for example, by a car radio, a compact disc reader, a television receiver, a hifi system or by other stereophonic sound sources. The system may include voice recognition which permits a user to give voiced commands for controlling notably the sound source.
This signal transmission system may in turn relate to a teleconference system which comprises a transmitting station which communicates with a receiving station for which stations the conversations captured in the transmitting station are to be recovered in the receiving station without degradation.
This signal transmission system may also relate to systems for which radio broadcast signals arrive by radio link in the form of mixtures on antennas, the radio broadcast signals being locally interfered by noise sources.
2. Description of the Related Art
By way of example, let us consider the case where the wanted signal is a speech signal coming from a person.
A first situation appears in the case of the transmission of conversations via teleconferencing. A microphone installed in a transmitting station captures the voices as well as the ambient noise, and all the sounds thus captured are transmitted to the receiving station. Evidently, the sounds broadcast by loudspeakers situated in the transmitting station and coming from the receiving station, will also be captured and then broadcast to the receiving station and cause undesirable echoes. A solution restricted to certain types of signals is revealed in the document entitled: "Stereophonic Acoustic Echo Cancellation--An Overview of the Fundamental Problem" by M. M. Sondhi, D. R. Morgan, J. L. Hall, IEEE Signal Processing Letters, Vol. 2, No. 8, 1995, pp. 148-151.
None the less, when the loudspeakers broadcast stereophonic sounds, no satisfactory technique is known which permits correctly isolating the person's voice expressed in the microphone.
Another situation occurs in the case where the voice to be captured is that of a driver who expresses himself in a microphone installed in an automobile over the past few years, there have been developed possibilities for the driver to have voice control of equipment inside an automobile. The object of this is to set the driver free from movements he has to make to effect certain settings or to have certain controls in the automobile itself. It is thus necessary, in a first period to recognize the voice message pronounced by the driver and then, in a second period, to decode this voice message and extract therefrom commands intended to influence the equipment. By placing several microphones inside the driver's compartment, there is achieved that the driver's voice is isolated and the commands it contains are decoded to take appropriate action. But the automobile is a considerably noisy environment where known techniques are not satisfactory, notably, when the driver's compartment contains loudspeakers which broadcast stereophonic sounds. Each time, mixed signals contain mutually correlated signals, it is very difficult to separate them and also to separate other signals that form the mixed signal.